The light-emitting layer of an organic electroluminescence device OLED is mainly made of fluorescent material, or phosphorescent material, or mixed fluorescent and phosphorescent material. The LED display unit consists of three kinds of red, green, blue pixels, and when a top-emitting OLED device structure is used, because the three kinds of pixels have different light-emitting wavelengths, the thicknesses of the light-emitting layers would have certain differences. Usually, an optical compensation layer is utilized to modify the thickness of a light-emitting layer, the thickness of the optical compensation layer can be over 100 nm, so the optical compensation layer needs to have very good electrical charge transfer rate, in order to ensure that the device has the characteristics of low voltage and high efficiency.
The material used to make the existing optical compensation layer has a high triplet-state energy level, but often has low charge transfer rate and thus cannot be made to be thick enough, therefore, as an optical compensation layer, it has a high drive voltage. In another aspect, material with a high charge transfer rate often has a low triplet-state energy level, which adversely affects the efficiency of green-light devices. Currently, the optical compensation layer is arranged between HIL and HTL, and is made of material with a high hole transfer rate (1.5-2 times of the transfer rate of NPB), although such arrangement alleviates the thickness increase of the organic layer to a certain extent and does not adversely affect the drive voltage of the organic light-emitting device, it does not take the special electric characteristic requirements of different light-emitting material into consideration, and cannot effectively increase the efficiency of the organic light-emitting device and reduce the power consumption of the display device.
The patent literature CN201210395191.7 of Samsung discloses an electroluminescence device, as shown in FIG. 1, it sequentially comprises a substrate 110, a first electrode 120, a hole injection layer 130, a hole transport layer 140, a buffer layer 150, a light-emitting layer 160, an electron transport layer 170, an electron injection layer 180 and a second electrode 190. The hole transport layer 140 consists of sequentially deposited layers of a first charge generation layer 141, a first mixed layer 142, a second charge generation layer 143 and a second mixed layer 144. The first charge generation layer 141 can be made of a mixture that contains a first compound and a second compound and is doped with a first charge generation material; the first mixed layer 142 can be made of a mixture that contains the first compound and the second compound; the second charge generation layer 143 can be made of a mixture that contains a third compound and a fourth compound and is doped with a second charge generation material; the second mixed layer 144 can be made of a mixture that contains the third compound and the fourth compound, in this aspect, the third compound and the fourth compound has a weigh ratio of 6:4 to 8:2. In this patent, the charge generation layer cannot provide an effective function for blocking excitons, so the buffer layer is required.
The patent literature CN200510077967.0 discloses an electroluminescence device, as shown in FIG. 2, a second hole transport layer 18-2 is arranged upon a first hole transport layer 18-1 in a green-light pixel area 200; the second hole transport layer 18-2 and a third hole transport layer 18-3 are arranged upon the first hole transport layer 18-1 in a red-light pixel area 300. The first hole transport layer 18-1, the second hole transport layer 18-2 and the third hole transport layer 18-3 can be made of different materials, but these hole transport layers are made of the same material. Although this patent discloses utilization of hole transport layers with a mixed structure to increase light-emitting efficiency and such arrangement alleviates the thickness of the light-emitting layer to a certain extent, the HTL material suitable to make the green-light optical compensation layer is still required to have a high triplet-state energy level TI with its HOMO energy level ≤−5.5 eV, and this kind of material often has a low charge transfer rate and thus cannot be made to be thick enough, therefore, this device has a high drive voltage.